Position determining method and apparatus



Nov. 20, 1962 w. R. BAILEY ETAL 3,054,897

POSITION DETERMINING METHOD AND APPARATUS Filed March 5, 1956 INVENTORS W/L sue/v A. 614/4 5; *4: ROBERT 6. flocw 2522M, zm, M /M ATTORNEYS Nov. 20, 1962 w, BAILEY ETAL 3,064,897

POSITION DETERMINING METHOD AND APPARATUS Filed March 5, 1956 5 Sheets-Sheet 2 4 l9 l7 l l I H 7 l8 u HY:

TO PULSE COUNT ING CIRCUITS I N VEN TOR S VV/l. sue/v 5AM E) 0 ROBERT 6. #001 ATTORNEYS Nov. 20, 196 w. R. BAILEY ETAL 3,064,897

POSITION DETERMINING METHOD AND APPARATUS Filed March 5, 1956 5 Sheets-Sheet 3 F T T "7'; T I 7| I WAVEFORM DIFFEREN- DETECTOR AMPL|F|ER SHAPER TIATOR DEcoDER I R I07 72 I SPIN MOTORS 86 87 75 no AND POSITION 1- REGISTER I I TAKE-OFFS i- I CLEAR REGISTER I CIRCUIT I l I- T 78a I I CONSTANT FREQUENCY T 840 I I I PULSE GENERATORS I OFF I I I CIRCUIT I I59 I \79 I- 76b-\ I I I 77bON II08 I coDER I GATE OFFI I I I I I CIRCUIT I I 8 3 I 78b I I I 1 ON I 6n I I SOURCE GATE 84" I I GENERATOR I CIRCUIT OFF I I I "Lrzhm sln I 78D f UNIT A I II II I I I COMPUTER COUNTIFIRS 80 I 820 I 9 b I I- J 97G I 04 960 II I 97b f ANGLE AND I DISTANCE PULSE COUNTING CIRCUIT COMPUTER UNIT 8 I 3 95 A00 9| 93 9 9 o L ANGLE DETEcToR *P SITION fi 92,\ COMPARAT0R gONTROL lOl I b JI ANGLE AND 94 DISTANC I lIOb I09 READOU? To DETECTOR A SPIN MOTOR \IIOO INVENTORS FIG.5

BY MMW W ATTORNEYS 1962 w. R. BAILEY ETAL 3,064,897

POSITION DETERMINING METHOD AND APPARATUS Filed March 5, 1956 5 Sheets-Sheet 4 F 7 PULSE COUNTING PULSE COUNTING C|RCU|T CIRCUIT I 38 limo TE '1 I (TIAB) I IANGLE COUNTER AND ll TRANSFER DEVICEJ I I I I04 PARALLACTIC ANGLE (TIBA) I I II COUNTER I PARALLACTIC I I I IANGLE COUNTERAND I22 l2| I26 lfifNiFiRlJallgEi II k DELAY TRANSFER 7 NETWORK I l43b I II l I l I I29 lzvv I I V r 2 l44b l SEQUENTIAL l 4 I I GATE READOUT l l I30 -|33 I44n 1 DELAY SELSYN RANGE l NETWORK) FINDER I32 I I |3| 99 I00 L J FIG.6

INVENTORS V/L sue/v R 5/1/45) '0 ROBERT 6. #001 wwwa ATTORNEYS @EQEH 3,64,897 Patented Nov. 20, .1 962 I g 3,364,897 'PQSITION DETERMKNENG METHGD AND V APPARATUS Wilbum Re Baiieyand itobert G. Hoch, Dallas, Tex.,i-a=- signers to Texas Instrumeritslncorporated, Dalias, Tex., a corporation of Deiaware Filed Mar. 5, 1955, seL-l 0. 559,579 .liClaims. (Q1. 235190) This inventionv relates to a passive system for determining the position of radiant energy producing objects. More 'p'articularly,this invention relates to a passive syste'mfor determining thebearing'distance and elevation of infraredradiation.producingobjects such as, for exam'ple,aircraft, ships, vehicles, and missiles. As used hereinftheword passive denotesa system which de- -tects radiant'energytransmittedfrom or emitted by an external source.

For many purposes, it is essential to be able to detect the presence ofaircraft, ships, vehicles, or otherobjects. Thus, in navigatinga craft on land, at sea-or in the air, thenavigator'needsto be able to observe-his position relative toother objects at all times. In times of warfare, pilots, ship at sea and ground crews must be able todetect the approach of other aircraft, ships, vehicles "and the various types of missiles.

v Under certain condi- -ditions, visible light waves may be employed for'observation and detection purposes but in case of dense fog,

adverse atmospheric COI'ldlilOIlS,OI the-lack of radiated or reflected visible light waves, such waves are not available for these purposes. Infrared waves overcome the-limitations of visible light waves in atleast two Ways. First, infrared'waves atcertain frequencies are .able to penetrate fog and, as aconsequence, provide detect-able information under adverse weatherconditions. And secnd, infraredwaves are produced -by heated objects such as aircraft engines and thesrnoke-stacks of-ships and thus infrared radiation is available even though there may 1 be no 1 visiblelight waves.

-In the prior arLsystems-utilizing radiant energy in the form of infraredwaves are known for measuring distance,

bearing, and elevation. One such systemhas-used infrared wavesto measure distance in'the forrncf elevation 'of an aircraft above the-ground. In the system, alig ht source, a major-portion of whose spectrale'nergy is-within theiinfraredrange, is. fixed at one portion of the aircraft .tosend a slender directive beam of spectral encr-gy to .the ground. .Located along a line'constituting the base of .a triangle, "and at a known distance from -the-' highly directive light beam is a. rotating angle measuring device.

Since the angleof the beam ofirifrared light is fixed with respect to th'e base line and thel'length' of the base line: is

known, the elevation of the aircraft above the groundiis then a direct trigonometric function of the infrared wave reflection angle measured by the rotatingangle measuring device. .In another system for indicating bearing, for instance the bearing of a ship, a-pairofthermo couples is located at the focus of a-re'fiector. The image of the source of infrared radiation formed-at the focus is of -suflicient size to fall on and energize the two thermorcouples to an equal extent when the reflector is; pointed -ing deviations from the correct elevationof the source.

.Thedisadvantage of the first mentioned systemzis,'rof

2 course, that it is'capableof measuring'only' the eleva- "tion of the aircraft above the'ground and'can'not be'used to measure the distance to other infrared .emitting'objects. In"the lattefnientiondisystem, the arrangement is capable of indicating only deviations in hearing or elevation from a source of radiant energy "and cannot be used to measure'either 'thedistanceto the source or the elevation, bearing -anddistanceof other infrared emitting objects.

In the present invention, a passive? systenfis provided for determining the bearing; distance and'elevation of radiant energyproducingcbjects by cletecting radiations emitted therefrom. A'lthoughthe system finds itsmost particular use in detecting infraredradiations,'it"is"n'ot to beconsideredaslimited" thereto since wavelengths in the visible light andshorter'wavelength regions may be utilized'withinth'eteachings of thisinven'tio'n. In the detection of radiations, two detectors are usedeach compi'isin'g an optical" system for-focusing radiantenergy'npon a photo-resistive or' a'photo-voltaic semiconductor. The detectors are gimbal mounted to "permitcon'stant speed -rotation about their vertical axes as well as angular movement about 'their' horizontal axes. T he detectors are spaced apart from 'ea ch other to establish-'afbase line and positioned therebetween-isa reference source of radiant "energy.

Whenevenradiant energy is detected during thescanning operation; 'thedetectors individually produce pulses which initiate"constant frequency pulse countingoperations. The pulse counting-'operationscontinue from "the point of detection until the detectors haverotate'dto-the "base A l-ine -and i their optical systems directly face the referenee source. "Then, a coded pulse from the source, through appropriate circuitry, stops "the piilse counting operations. lThe numberof; pulses counted in each separate operation is a direct' rneasu-re'of the timef rotation an'd, -thus, the number of pulses-is representativeof the angle at thedetectorbetween the line of: detection andthe base line. Knowing the length of thebase line'and the two included angles; the re'rriair-ring angle of the triangle defined' -is determinedas a voltage in a polar coordinatetype .cornpnter. The voltageof the:arigle, known "herein as the paralla'ctic anggle, represents thediameter'of a circle w-hich :passes through a sourcecf-infrared r adiation-andone .of v the detectors. ..A 'selsynat the detector" measures the cosine :of.the =angle between a line perpendicular to "the :basev line at lthe' detector and the line of radiation fi om the infrared source in terms of voltage. Then by cornbining the diameter ofithe circle voltage-and the cosine of the angle voltage, :a voltage-is produced whichmeastires the distance to the .source. The-cosine ofthean gle voltage and the distance "voltage rnay .be applied i to I a :suitable bearing -angle :and distance readout device.

In addition to the cosine 'of the angle.selsyn, anoth er -=selsyn is mounted on the detector choseniforthe-distance "computation. This selsyn. is-e'tlignedswiththe horizontal tilt axisand producesfa sine wave'voltage corresponding to the cyclical tilt of the detector which voltageis applied to the vertical deflecting plates of a cathode ray' tube. The cosine of the angle :selsyn, mounted on thedetector vertical spin axis, feeds its outputvoltage to' theihorizontal deflecting plates of the cathodelray tubeiin apath separate from the computer. The same pulse which initiates the pulse counting operation "is fed to the cathode ray tube to intensity modulate the cathode and thereby pro duce' a spoton the screen indicative of the elevation angle and the bearingangle. Thebearing. angle is repeated as an aid in correlating the elevation, bearing-and distance information concerning a; radiating object.

:Accordingly, it is anobject ofthis invention to provide a passive position determining systemsuitablefor militai'y 'as well as non-militaryuse.

frared radiations at each of at least two constant speed rotating detectors.

It is another object of this invention to measure the angle between two separated radially extending lines as a function of the time required for a constant speed detector to rotate from one line to the other.

It is a further object of this invention to measure angles as a function of time by counting the number of constant frequency pulses generated over a period equal to the time function of said angle.

It is a still further object of this invention to provide the distance of an infrared radiating object by a computation technique which includes determining the parallactic angle.

It is a still further object of this invention to compute the distance of an infrared radiating object by techniques involving the polar coordinates of a circle.

The above objects will be clarified and other objects made known from the following discussion when taken in conjunction with the drawings in which:

FIGURE 1 is a partial schematic illustrating a preferred arrangement for the components used in determining the position of infrared radiating objects;

FIGURE 2 is a schematic representation of the radiation focusing optical system, the photo-conductive cell and the A.-C. voltage circuit comprising the detector of this invention;

FIGURE 3 is an enlarged view of the radiation chopping system used in conjunction with the photoconductive device and A.-C. voltage circuit of FIGURE 2;

FIGURE 4 is a view in perspective of a gimbal mounted detector suitable for use in aircraft;

FIGURE 5 is a block diagram of the circuit components required in conjunction with the detector of FIG- URE 2 to compute and indicate the distance, bearing and elevation of infrared radiating objects;

FIGURE 6 is a block diagram of the circuit components comprising the polar coordinate type computer in the preferred embodiment of this invention; and

FIGURE 7 is a polar coordinate plot of a family of circles for various values of constant parallactic angle and illustrates that the diameters of the circles are determined by and are inversely proportional to the size of the parallactic angles.

Referring now to the drawings, FIGURE 1 represents a partial schematic of the component arrangement used in the present invention to determine the bearing and distance of infrared producing objects. In FIGURE 1, the detectors A and B are separated at a distance from each other, the letter at designating a straight line and the distance between A and B. A reference source of radiant energy S is positioned on the line d intermediate A and B. Preferably, source S is positioned at the midpoint between A and B thus dividing the line d into the equal segments d and d Detector A is mounted in agimbal arrangement which permits the optical system to scan in elevation as it is being rotated in a clockwise direction. Detector B is mounted in like manner to detector A and also rotates in a clockwise direction. As the detectors rotate, infrared waves from sources T T T are received and angles are measured from the line of detection to the base line d. Thus, detector A measures angles T AB, T AB T AB and detector B measures angle T BA, T BA T BA.

The detector optical and electrical system which initiates the angle measurement operation of this invention is shown schematically in FIGURE 2. In FIGURE 2, the infrared radiating from some source, for example T first fall upon the optical window 12. Optical window 12 is a silicon crystal formed in the shape of a hemisphere and may be of a size up to six inches in diameter and have a thickness of 0.2 inch. Window 12 is bonded to a closure member 15, only a fragment of which is shown. Silicon provides an ideal window for infrared radiations since it is opaque to ultra-violet, visible light and very near infrared waves up to 1.2 microns in length and is essentially transparent to the longer wavelengths of infrared. However, should it be desired to detect radiant energy in the very near infrared, visible or ultra-violet wavelength regions, some optical window such as glass can be substituted for the silicon window 12. Silicon transmits approximately 53% to 55% of the radiation Wavelengths between 1.2 microns and 7 microns, the amount transmitted depending upon the wavelength of the incident radiations and the temperature of the silicon. In order to maximize the transmission characteristics of silicon over the wavelengths of interest, anti-reflection coatings 13 and 14 are coated on the silicon window 12. Such coatings may be of arsenic trisulfide or titanium dioxide and, when coated on the silicon in thicknesses of wavelength or multiplies of a wavelength at the desired frequency, a transmission efficiency of 90% or greater may be achieved for wavelengths over a 2 micron band and of about for Wavelengths over a 4 micron band.

The rays 10 and 11 are transmitted in a refracted path through window 12 and are collected by spherical mirror 16. From the spherical mirror, the rays are reflected first to plane mirror 17 and thence to plane mirror 18. Plane mirror 18 is mounted in the open center section 19 of spherical mirror 16. The angle at which mirror 18 is mounted is such that infrared waves are focused through an aperture 20 upon a recticle 22 so as to be slightly off center from the vertical spin axis 25 of the detector. Aperture 20 is formed in blank 21 with a diameter of 0.25 inch thus limiting the radiations received to those falling within a beam width angle of 2.

The reticle 22, in order to be transparent to infrared radiations, is a silicon disc and constitutes a radiation chopping or modulating device. As shown in FIGURE 3, reticle 22 is coated in such a manner as to produce opaque areas 23 alternating with radiation transparent areas 24. Although the opaque areas are shown as be ing wedge-shaped, the areas 23 may take any shape or form which will modulate the radiations transmitted through the reticle. As has been mentioned above, the radiations focused upon reticle 22 by mirror 18 are off center from the vertical axis of rotation 25. The radiations are focused primarily on the point indicated by dot 27 but, due to the phenomenon of spherical aberration resulting from mirror 16, the radiations spread out over an area indicated by 28. When the optical components are rotated relatively to reticle 22 about the axis 25, the incident radiations follow a path described by the circle 29. It can thus be seen that the radiations fall on alternate transparent and opaque areas.

The radiations transmitted through the reticle 22 fall on the photo-sensitive semiconductor or detector cell 30. Photo-sensitive semiconductors, which may be either photo-voltaic or photo-resistive in type, have the property that, in the presence of darkness, they become virtual insulators but, in the presence of light, they become moderate conductors. Photo-voltaic semiconductors, typified by indium antimonide photocells, contain a p-n junction and, upon the incidence of radiations, a potential difference is created across the p-n junction. Photo-resistive devices, comprised for example of lead telluride, exhibit the property of decreased resistance in the presence of radiations. As used herein, the detector cell 30 exhibits the properties of a photo-resistive semiconductor and is biased by battery 31. Connected in series with semiconductor 30 is a resistor 32. The modulated radiations from reticle 22 incident upon semi- .conductor alternatelylincrease and :decrease its resist- :anceandthus an A.-C. voltageris.developedacross.resistor 32. This .A.-iC. .ontput vvoltage -isfed through the -D.-C. tblocking .condenser 33 to. a. pulse counting circuit which. is described hereinafter .-.in -connection with FIG- .LURE 5. -Leads 35 .and.36.serve,. respectively, to feed the .bias .voltage .to. and -the .output' voltagefrom .cell..30.

'A.detector,..constructed in accordance with the .princi- -ples ;described in'uFlGURE. 2, 'may =be.installed-in;an .aircraft, on ships, -vehicles, .or.in aistationary groundin- ..stallation-as-an:integral component. of. a system for deter- .mining the position-of-radiation-emittingmbjects. .For a .lletector. embodiment suitable!forinstallation in aircraft, referencenis :now madeiothe 'perspective view of- FIG- .URE-A. -SimilaLcomponentsin.EIGURESQ, .-3,..and 4 -.carry like numbers.

.A dishedcylindrical member Afiiencloses spherical-mirror16. .Dependingfrommember-.40 arestruts-Ala, 41b, and 410 :serving to isupportmirror .17. Aperture '20 is formed inthe tunderside of member" in line withthe vertical axis of rotation vI25. lConcentricabout #vertical =axis.25, thelinner raceofball bearing 43-isfixed to the underside ofniember 40whilelthe outer raceof thebear- .ing is secure'd-to-a legfiofthe rU=shaped yertical yoke 42.

Bearing 43 is constructed so as to providean internal- .cavityarea-44. Eavitvarea 44-contains reticleZZZ and detector reel] .30 and permits them to :remain vin a fixed but stationaryposition relativetothe optical components. :In .line .with vthe vertical laxis, rod 45 is fixed to the -uppeiiside of. member 40. Rod 45 is secured to the inner .race;of.ball-bearing-46 and the outer-race of' ball bearing 46...is.secured-to the upper leg of yoke 42. The -upper end .of zyoke 42supports the detectorspin motor '47. The aoutputashaft148'of-motor 47 is supported 'by abushing 49 idependinglfrom the .upper.legiof yoke 42 and.a bevel gear 50-iskeyed. to theiendof output shaft 48. Bevel-gear-50 engages. a matching bevel gear 51 .keyed to rod- 45.

LThe ivertical yoke and optical :eomponent assembly is afiixed'to and supported at-theicenter line-ofthe-horizontaLaxiszyokeSS. .Thebuter races of ball bearings57 and .59 are :secured to the vertically extending legs of the LU-s'haped :horizontal yoke55. -Shafts 56 and 5 8-are :secured toithe inner races of ball bearings.57 and 59, respectively,.sandithe shafts :56 and 58am afiixed'to the enclosing cylinder 15 at the horizontal tilt axis 2.6. The "optical window 1 2 provides:the-closure for cylinder 15. Suitable-means for tilting the assembly about the 'horizon- -tal tilt axis 26is provided byaservo-motor drive 60 fixed to the cylinder 15. A disc 6 2-is-'keyed to the-outputshaft 61 ofservo-drive-60 and eccentrically connected thereto isaconnecting rod 63. Connections-Maud 65 rotatably connect rod 63 to the disc-62 and the-horizontal yoke 55, respectively.

.From FIGURE 4, it can' be'seen that the optical components are free to spin about vertical axis 25 While reticle 22 and detector cell 30 are allowed to remain stationary but.in fixed alignment with the optical components. The elevation scanning 'feature of the optical system is provided by the horizontal yoke and servodriven eccentric Whichtilts the optical components about the horizontal axis'26. The preferred motor speeds'for the detector-of this invention provide a-spin rateof the optical components about vertical axis 25 0f 600 r.p.m. and a cyclical scan rate abouthorizontal axis 26 of 30 cycles per minute. The embodiment of FIGURE 4 is very advantageous for .airbornelinstallations because of .the small wind .resistance and drag :created by :the idetec- ;tor. .However, theJimited scanthus provided by the stationary window.12 maybeundesirable for ground and .ship board installations. .Therefore,for ground .andship boardinstallations, the optical-window'and cylindercould be made to rotate as an integral unit with-the optical components and the gimbal mounting arrangement would then be "externally-connectedto the cylinder'lS.

Referring now to FIGURE 5, a block diagram is 1, :Radiation Laboratory ..Series. conducting to a non=conducting state,'the second.halfof 40 l :showno'f the circuitry required in conjunction with-the detectors .ofFlGURE 2 to compute and derive the distance, bearing and "elevation of an infrared emitting object. :The description of FIGURE 5 is primarily in terms of only the detector A since the pulse counting circuitsfor zboth detectors Av and B, for the most part, are identical. .Furthen'the circuits and components involved .in determining the space coordinates of am object are .well-knownzin the computerxand radar arts and it .is, .therefore, .deemed unnecessary for the purposes of this inventiontoshow the circuitsand components in detail.

Themodulated. A.-.C. output voltage,produced by detectonAupon receiving infrared'radiations from source T is fedtothe pulsecountingcircuit.37shown enclosed by dotted lines. The 'A.-.C. signalsare first amplified in an .amplifier .70 and, afteramplification, are fed to a waveformshaper7l. 'Waveform shaper 71 may be either .a .conventionalsquaring amplifier or a diodelimiting circuit. 'Afterbeing shaped toa rectangular waveform,

lthe waves are fedto a diiferentiator 72 where a conven- 'tional resistance-capacitance.differentiating network proequal toltheznumber .of'infrared sources to .be accepted :during :anyone'horizontalscan of .the detector. .Initially,

. prior to the reception .of. a" signalpulse, the input side. of

.each flip-flop isiin ac'nonaconducting state. The firstpositivepulseirom the. difierentiator circuit 72 causes the in put :of .the.first .flip-flopstage to "change from a nonconducting state to :a conducting .state and the second half of the :first :stagethen becomesnoneconducting. For

a discussion of flip-flop circuits, see section 13.7, volume On changing lfIOlTl a the first stagefiip-flopgenerates a positive pulse inits plate .circuit as the :plate .current .ceases. .This positive .pulse isapplied through lead 76a to gate circuit 77a which may :also Lbea flip-flop-circuit. The input .side 10f gate circult 77a is normally conductingand the positive I pulse flips athecircuit to .cause the second half to become .con-

lductive. The second -half of gate .circuit77a is then in condition Ito receive pulses from :the .constant frequency .-pulse generator 79 through lead 78a and permits these pulses tofeedithrough leads-8l'a to the.counter.circuit182a.

.Countencircuits'82 .consistof a series of conventional binary sealers connected .in. cascade.

As detector A continues-its horizontal-scan, radiations rnay be-detected froma second source T Inthe'manner'described above, .the :radiations from T result-in a positive:pulse beingfed-to register 75. -Due to the first positive pulse, the input half of the firstfiip-fiop stage Was-changed-to a conductingsstate and the second'half changed to anon-conducting state. The second .pulse received-causes the conducting states to reverse thus making the first half non-conducting and the second .half

conducting. Thepositiveipulse from the plate circuit of an'infrared source T the process is entirely similar and results in apulseifromthe nth stage flip-flop in register through lead 7611 to gate circuit 7.7m. 'Gate circuit 7711 becomes .conductive and passes constant frequency source S located on line d.

pulses through leads 78n and Sin to counter circuit 8211.

Constant frequency pulse generator 79 constitutes part of the circuit 39, the circuit portion of the position determining system which is common to both pulse counting circuits 37 and 38. In this invention, pulses are produced by generator 79 at a frequency rate of 12.96 megacycles per second. When compared with a rotational speed for the detectors of 600 r.p.m., this frequency corresponds to one pulse for each second of angle scanned by the detectors. However, this pulse frequency generation constitutes no limiting factor on this invention since the pulses may be generated at any desired frequency. The constant frequency pulse path to the pulse counting circuit 38 for detector B is by means of lead 80.

All angles to radiation emitting objects T T T are measured with respect to line d between the detectors A and B. Therefore, as detector A rotates to a position whereby radiations are received along line d, gate circuits 77 must be closed to stop the fiow of constant frequency pulses to the counter circuits 82. This is accomplished by first receiving at detector A a signal from the reference infrared In order to eliminate ambiguity as to Whether a received signal is from a radiating object T T or from source S, the reference source signal is provided in coded form through a coder 83. Although a number of coding techniques may be used including pulse width and pulse position modulation, the preferred technique is to provide an interlock with the detector scan position. In this technique, the reference source radiates energy only during a narrow azimuth range. The circuit whereby this is accomplished consists of a commutator fixed to the vertical axis of the detector scan system in such a manner that the source is permitted to radiate only when the detector scan position is in the region of, for example, 5 to either side of the line d.

Coder 83 modulates pulses from pulse generator 79 so that the output of source generator S is limited to a series of infrared radiataion pulses having a fixed duration and fixed repetition rate. The pulse from' sources are processed by the detector and the amplifier, waveform shaper and differentiator circuits in the same manner as described for radiations from T T Instead of passing directly to the gate circuits 77, however, the pulse code group is passed to the decoder 73 through a gate which is energized only when the commutator is in the closed position. Decoder 73 consists primarily of a delay circuit and a coincidence circuit. The delay time is equal to the spacing between pulses. The first pulse of the series is delayed for one period and is applied in synchronism with the second pulse to the coincidence circuit. When coincidence occurs, a pulse is put out by the coincidence circuit. This output pulse is applied to gates 77a, 77b 7711 through leads 84a, 84b 8411 respectively, which flips the circuits so that the input sides are once more conductive and the second halves are non-conductive. At the instant of closing gates 77, the number of constant frequency pulses passed through the gates to the counter circuits 82 represents a direct measure of the angle between the line d and the line of radiation from each of the sources T T Simultaneously with the pulse to gates 77, the pulse from decoder 73 is fed through lead 85 to register clear circuit 86 and through lead 88 to the counter circuits 82. The pulse to register clear circuit 86 triggers a series of pulses which are fed through lead 87 to register 75. These series of pulses reset the flip-flop circuits to that the first half of all stages are in the non-conducting state and the'second halves are in the conducting state. The pulse applied through lead 88 to counter circuits 82 advances the number of pulses accumulated in each counter to its section of the angle and distance computer 98 through leads 96a, 96b 96n.

Pulse counting circuit 38 for detector B is identical to the pulse counting circuit 37 described for detector A. However, because detectors A and B rotate in the same direction and detector B will thus be displaced 180 when detector A is facing source S, the counter circuits of circuit 38 operate conversely to those of circuit 37 in order to permit angles to be read directly. To describe this operation, the counters in circuit 38 equivalent to the counters 82 in circuit 37 are preset with a number of pulses representative of an angle of 360 and are connected in the reverse direction to counters 82 so that pulses are subtracted from the number preset into the counters. Thus, whenever pulses indicating the presence of infrared sources T T T are received, gates 1 n in circuit 38 are opened and constant frequency pulses from pulse generator 79 will be fed through the gates to the counter circuits. The pulse counting (subtracting) continues in all circuits until detector B faces source S and then a coded pulse from S is fed to the gate circuits to stop the pulse counting (subtracting) operation. The same pulse from S is also fed to the counters and serves not only to advance the number of pulses remaining in each counter to computer 98, which pulses directly measure the angles to T T T,,, but also to preset the counters with the number of pulses representative of 360.

To insure that the angles at detectors A and B are being measured to line d as the common reference line, the pulse counting operations in circuits 37 and 38 must be stopped exactly one half revolution apart. This is accomplished by mounting 360 potentiometers to the vertical axes of detectors A and B. The voltage across each potentiometer at the zero commutator position is fed from detector A by lead 90 and from detector B by lead 91 to angle comparator 92. The voltage from detector A is delayed for a time equal to one half revolution by means in angle comparator 92 so that the voltages may be compared simultaneously. There the voltages are compared and, if a difference voltage exists, this voltage is fed to detector position control 93. The difference voltage is amplified and fed as a negative feedback voltage through lead 94 to the spin motor for detector A or through lead 95 to the spin motor for detector B. The negative feedback voltage serves either to increase or decreae the spin rate of the detectors and maintain their rotation at a constant 600 rpm.

Reference is now made to the block diagram of FIG- URE 6, which in conjunction with FIGURE 5, illustrates the components comprising computer 98. The principal purpose of computer 98 is to compute the distance to an infrared source and, for this purpose, is provided with polar coordinate functions. To explain, from the equation of a circle in terms of polar coordinates, it can be shown that for a constant parallactic angle, the distance to an infrared source will be on a circle having a diameter equal to the distance of the source if it were on a line of bearing '90 from theline between the detectors. Parallax may be defined as the difference in apparent direction of an object as seen from two different points and, as used herein, the parallactic angle is used to mean the angles at T T To illustrate that a constant parallactic angle defines a circle, FIGURE 7 shows a family of circles for various values of constant parallactic angles. The family of circles is drawn through an origin, described hereinafter as detector A, and it can be seen that the diameters of the circles vary inversely with the size of the parallactic angles.

- Thus, the diameter of the circle defined by the parallactic R=D cosine (T AB T AB90) where ,pulse comprising the parallactic angle. achieved by the transfer device 123 at the instant of 'therefiectingantenna and thus'to the distance d :R'=distance in feet at angle l AB T AB,

D=the diameter of the circle in feet, and T AB lT AB=the angle from alme of infrared radiation to ;the.line between detectors A and B.

The functions of the above equation areprovided by computer 96 in the following manner. The counted pulsestrom the counterfiZa or circuit.37 are received at parallactic angle counter 120 through lead 96a and the pulses .from the'like .counter in circuit 325 through lead 91a. Counter 120.is a binary subtractor preset with a number of pulses equivalent to anangie of 180. Since detector A leads detector B by a one-half revolution, the number of pulsesrepresenting angle T AB arereceived firstby counter 120 andsubtracted and then the'nun'rber of pulses representing. angle T BA iaresubtracted. The remaining number of pulses represents the parallactic angle at T which pulses are fedthrough lead 121 to transfer device 122. The coded .pulse from source 'S, which stops thepulse counting (subtracting) operation inrcircuit '38 and advancesthe pulses to computer 98, is fed through lead 124 to delay network 123. Afte-r'a delayin network 1123 .sufiicient to permit thepulses representing T BA to .besubtractedin counter 12%, the pulse 'is fed throughlead 125to transfer device 122 and through 'lead 126 to counter 120. This pulseipresetscounter 126 to a number of pulses equal to 180. However, in transfer device 122, thepulse initiatesa voltage summing operation which consists of adding the voltage from each The voltage counting the finalpulse is equivalent-to D, the diameter ofthe circle forthe'particularparallactic angle. The D voltage is then discharged through .lead 127 to sequential readout 128.

It shouldbe recognized that, in order to determine the parallactic anglein a computational method based on polar .coordinates, it is necessary to know the distance a. 'between detectorsAand B. Whenthe base length distances d and d are rigidly fixed, as in some airborne installations and in ground installations, the measurement of d and d may be made once 'and"set into computer '98 as a constant value. However, when it is necessary to measure the d distance, any one of several methods may be used but the following is preferred from the standpoint of accuracy. A miniature radio frequency transmitter is located at reference sources. A transmitter generates a continuous wave signal on "a convenient radiofrequency andfeeds this signal to an antenna. .A small tuned reflector mounted at detector A (not shown) reflects the radiated waves back to the transmitting antenna where a beat frequency is produced between the transmittediandreceivedwaves. This beat frequency-is passed'by an RF choke to the computer 102, shown inthe block .diagram of FIGURE 5. The frequency of the beat .noteis proportional to the distance from S to This beat note frequency is used to control the number of constant frequency pulses fed from generator 79.to computer 102' through lead 103 and from the computer 102 to computer 98 through lead 104. 'A similar arrangement is required to obtain the distance d between source S and detector B. The d distance computation correction may be achieved in counter 120 by, for example, addingpulses to the number of pulses representative of 180 should .the d distance exceed a fixed distance and "by subtracting pulses'should the d distance decrease be- 1 equal to cosine(T AB90 is permitted to ,pass.

1% the desired angle. The output from=sel syn 105 is fed through leads 106 and 1tl6ato, gate 129 in the computer 98. As has been described above, :whenever an infrared source is detected, detector A produces a modulatedA-C.

,5 'voltage which is amplified,ishaped and differentiated in 15 the-diameter of the circle for the parallactic. angle at T is fed through lead 133 to the selsyn range 'finder132. Selsyn range finder 132 consists of.-a selsyn -revolvingin synchronism with the scan head. The D voltage is aptplied to the selsyn and, when the angle -voltage, cosine (RAB-90) from lead 131 is applied to the selsyn,

the output voltage-.obtainedtfrom theiselsyn'is then-D cosine(T AB.90), which .is very substantially the .R voltager'or thedistance r totthe infrared radiatingsource T "-a'siindicated in FIGURE .1. The distance voltage-R is then applied to the analog angle-and distancereadout 101 through lead 100 and,: simultaneouslytherewith,lead '99 connected to lead 131 between delay .network130 andirange finder 132, feeds-the angle Voltagetoreadout "101. Readout 101may be a cathode ray tube with X.Y

coordinates of-=bearing-angle and .distance .orsome other positioning apparatus sensitive to voltages.

To permitsuccessive computation of:R voltages,..repre- ".senting distances-r r to-infraredsources T :T ,;p-arallactic angle counters: and transfer vdevices 143b v 14311 are provided. Circuit 1431b receives ,pulses through leads 96b and 97b and circuit 143m receives impulses through. leads 96n and 9721. Circuits 143i) .and 143m provide .-.D woltages which are then fed through .leads 1 14b and -144n-to the sequential readout .128. .Se-

; -quential readout128 applies the D voltages sequentially to the'selsyn-range finder .132 where the vRdistance voltage :to each infraredsource T are-computedin .the :manner described :above.

Although the computer logic in the preferred :embodiment of this invention utilizes polar coordinate functions,

:it:is not. to be considered as limited thereto. For example, "since-the detectors-A and B measure angles and thedepth of the included side d -is.-known, a computer provided with functions according to-the lawof sines could be used to compute ther .-r distancesto the infraredsources In orderto show the elevation of the infrared sources T T and also 'to correlate the distances and bearingsshown on readout .101 with'theelevationsto the infraredsources, FIGURES shows selsyn .107 attached .to

the tilt aXis ofdetector A in addition vto the selsyn .105. The output from selsyn 105, connected tothe verticalaxis, is fed through lead 106bto'the horizontal-plates oftcathode ray tube 109. The output of selsyn 107 is a sine 6O wave voltage which is fed through lead 108 tothe vertical plates of tube 109. The samepositive pulsefrom difierentiator circuit 72, used to .open gate 129.. momentarily for .a particular infrared source, is simultaneously-fed by lead lltib to the cathode of tube .109. Thecathodelis intensity modulated by the pulse andemits electronspro- .ducing a spot on the screen indicative of .the:e1evation angle and bearingof the radiation emitting source. .For a discussion of elevation and bearing angle presentation on cathoderay tubes, see section 6.6, vol. .1, Radiation Laboratory Series.

.Itis apparent'thatithemethod and system .of the present invention, as described, is subject to considerable choiceinthe pulse counting and computer circuits, methods of computation, computer logic, wavelengths of radi- 7 5 antenergy to .be detected, and choice of materials, speeds and detector embodiments. Therefore, any such changes or modifications as fall within the scope of the appended claims are intended as part of this invention.

What is claimed is:

1. A system for determining the position of radiant energy sources including a first radiant energy detector rotating at constant speed, a second radiant energy detector rotating in synchronism with said first detector and separated therefrom, means measuring individually the angle at each of said first and second detectors from a source of radiant energy to a common baseline therebetween by counting constant frequency pulses over the time period of rotation of each said first and second detectors required to define said angles, and means utilizing said counted pulses representative of said angles and the distance separating said detectors in computing the dis tance to said radiant energy source.

2. A system for determining the position of radiant energy sources including a first radiant energy detector rotating at constant speed, a second radiant energy detector rotating in synchronism with said first detector and separated therefrom, a plurality of means measuring the angles at each of said first and second detectors from a plurality of radiant energy sources to a common baseline therebetween by counting the constant frequency pulses over the time period of rotation of said first and second detectors required to define each of said angles, and means utilizing said counted pulses representative of said angles and the distance separating said detectors in computing the distance to each of said radiant energy sources.

3. A system for determining the position of radiant energy sources including a first radiant energy detector rotating at constant speed, a second radiant energy detec tor rotating in synchronism with said first detector and separated therefrom, a fixed source of radiant energy po sitioned intermediate said detectors, means measuring the angle at said first detector between a line of radiation from a radiant energy source and a line from said fixed source, means measuring the angle at said second detector be tween a line of radiation from the radiant energy source and a line from said fixed source, and means wherein said measured angles and thedistance between said detectors are utilized in computing the distance to the source emit' ting said radiant energy.

4. A system for determining the position of radiant energy sources including a first radiant energy detector rotating at constant speed, a second radiant energy detec tor rotating in synchronism with said first detector and separated therefrom, a fixed source of radiant energy positioned intermediate said detectors, constant frequency pulse counting means measuring the angle at said first detector between a line of radiation from a radiant energy source and a line from said fixed source, constant frequency pulse counting means measuring the angle at said second detector between a line of radiation from the radiant energy source and a line from said fixed source, and means wherein said counted pulses representative of said angles and the distance between said detectors are used in computing the distance to the source emitting said radiant energy.

5. A system for determining the position of radiant energy sources including a first radiant energy detector rotating at constant speed, a second radiant energy detector rotating in synchronism with said first detector and separated therefrom, a fixed source of radiant energy positioned intermediate said detectors, a plurality of constant frequency pulse counting means measuring the angles at said first detector between lines of radiation from a plu rality of radiant energy sources and the line from said fixed source, a plurality of constant frequency pulse counting means measuring the angles at said second detector between lines of radiation from said plurality of radiant energy sources and the line from said fixed source, and means wherein the counted pulses representative of said angles a d 16 di t nce between said detectors are used in computing the distances to said sources emitting said radiant energy.

6. A system for determining the position of radiant energy sources including a first radiant energy detector rotating at constant speed, a second radiant energy detector rotating in synchronism with said first detector" and separated therefrom, a fixed source of radiant energy positioned intermediate said detectors, constant frequency pulse counting means measuring the angle at said first detector between a line of radiation from a radiant energy source and a line from said fixed source, constant frequency pulse counting means measuring the angle at said second detector between a line of radiation from the radiant energy source and a line from said fixed source, means at one of said detectors producing an indication of the bearing angle to said radiant energy source and computer means utilizing said bearing angle indication, said counted pulses representative of said angles and the distance between said detectors in computing the distance to the source emitting said radiant energy.

7. A system for determining the position of radiant energy sources including a first radiant energy detector rotating at constant speed, a second radiant energy detector rotating in synchronism with said first detector and separated therefrom, a fixed source of radiant energy positioned intermediate said detectors, constant frequency pulse counting means measuring the angle at said first detector between a line of radiation from a radiant energy source and a line from said fixed source, constant frequency pulse counting means measuring the angle at said second detector between a line of radiation from the radiant energy source and a line from said fixed source, means at one of said detectors producing an indication of the bearing angle to said radiant energy source, computer means utilizing said bearing angle indication, said counted pulses representative of said angles and the distance between said detectors in computing the distance to the source emitting said radiant energy, and readout means receiving said bearing angle indication and said computed distance.

8. A system for determining the position of radiant energy sources including a first radiant energy detector rotating at constant speed, a second radiant energy detector rotating in synchronism with said first detector and separated therefrom, a fixed source of radiant energy positioned intermediate said detectors, a plurality of constant frequency pulse counting means measuring the angles at said first detector between lines of radiation from a plurality of radiant energy sources and the line from said fixed source, a plurality of constant frequency pulse counting means measuring the angles at said second detector between lines of radiation from said plurality of radiant energy sources and the line from said fixed source, means at one of said detectors producing indications of the bearing angle to each of said plurality of radiant energy sources, computer means utilizing said bearing angle indications, said counted pulses representative of said angles, and the distance between said detectors sequentially in computing the distance to each of said radiant energy emitting sources, and readout means receiving said bearing angle indications and said computed distances.

9. A system for determining the position of radiant energy sources including a first radiant energy detector rotating at constant speed, a second radiant energy detector rotating in synchronism with'said first detector and separated therefrom, a fixed source of radiant energy positioned intermediate said detectors, a plurality of constant frequency pulse counting means measuring the angles at said first detector'between lines of radiation from a plurality of radiant energy sources and the line from said fixed source, a plurality of constant frequency pulse counting means measuring the angles at said second detector between lines of radiation from said plurality of radiant energy sources and the line from said fixed source, means at one of said detectors producing indications of the bearing angle to each of said plurality of radiant energy sources, polar coordinate type computer means utilizing said bearing angle indications, said counted pulses representative of said angles, and the distance between said detectors sequentially in computing the distance to each of said radiant energy emitting sources, and readout means receiving said bearing angle indications and said computed distances.

10. A system for determining the position of radiant energy sources including a first radiant energy detector rotating at constant speed, a second radiant energy detector rotating in synchronism with said first detector and separated therefrom, a fixed source of radiant energy positioned intermediate said detectors, a plurality of constant frequency pulse counting means measuring the angles at said first detector 'between lines of radiation from a plurality of radiant energy sources and the line from said fixed source, a plurality of constant frequency pulse counting means measuring the angles at said second detector between lines of radiation from said plurality of radiant energy sources and the line from said fixed source, means at one of said detectors producing indications of the bearing angle to said plurality of radiant energy sources, means at said detectors to cyclically tilt said detectors during rotation, means mounted at said one de tector for obtaining indications of the elevation angle of said detector, a polar coordinate type computer utilizing said bearing angle indications, said counted pulses repre- 14 sentative of said angles and the distance "between said detectors in computing the distance to said radiation emitting sources, readout means for receiving said bearing angle indications and said computed distances, and means displaying said bearing angle and elevation angle indications of said radiation emitting sources.

'11. A system for determining the position of radiation emitting sources as defined in claim 10 wherein said counted pulses representative of said angles are converted in said polar coordinate type computer to analog form in computing the distances to radiation emitting sources.

12. A system for determining the position of radiation emitting sources as defined in claim 10 wherein said elevation angle and bearing angle indications and said computed distances to radiation emitting sources are presented to said readout means and said bearing angle and elevation angle displaying mean-s in analog form.

References Cited in the file of this patent UNITED STATES PATENTS 2,070,178 Pottenger et a1. Feb. 9, 1937 2,116,717 Scharlau May 10, 1938 2,246,496 Asbury June 24, 1941 2,489,222 Herbol-d Nov. 22, 1949 2,710,962 Fritze June 14, 1955 2,760,190 Henrici Aug. 21, 1956 2,830,487 Grifli-th Apr. 15, 1958 

